WordPress is being used by more than 60 million websites. Its is used by 58.6% of all the websites whose content management system we know. This is 24.9% of all websites on internet.
There are many wordpress themes available in market, some are free while some other are paid. Here is a breakup of themes being used as on Nov 2, 2015. This chart was taken from Builtwith.
Top in WordPress Theme · Week beginning Nov 02, 2015
I was surprised to see that Twenty Ten, Twenty Eleven, Twenty Twelve, Twenty Thirteen and Twenty Fourteen are still in use, while Twenty Fifteen could not secure a place in the list. (These all are free themes provided with WordPress).